﻿using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

namespace Sidvall.Data.Entity
{
	public static class PredicateBuilder
	{
		/// <summary>
		/// Creates a predicate that evaluates to true.
		/// </summary>
		[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1006:DoNotNestGenericTypesInMemberSignatures")]
		public static Expression<Func<T, bool>> True<T>() { return param => true; }

		/// <summary>
		/// Creates a predicate that evaluates to false.
		/// </summary>
		[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1006:DoNotNestGenericTypesInMemberSignatures")]
		public static Expression<Func<T, bool>> False<T>() { return param => false; }

		/// <summary>
		/// Creates a predicate expression from the specified lambda expression.
		/// </summary>
		[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1006:DoNotNestGenericTypesInMemberSignatures")]
		public static Expression<Func<T, bool>> Create<T>(Expression<Func<T, bool>> predicate) { return predicate; }

		/// <summary>
		/// Combines the first predicate with the second using the logical "and".
		/// </summary>
		[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1006:DoNotNestGenericTypesInMemberSignatures")]
		public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
		{
			return first.Compose(second, Expression.AndAlso);
		}

		/// <summary>
		/// Combines the first predicate with the second using the logical "or".
		/// </summary>
		[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1006:DoNotNestGenericTypesInMemberSignatures")]
		public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
		{
			return first.Compose(second, Expression.OrElse);
		}

		/// <summary>
		/// Negates the predicate.
		/// </summary>
		[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1011:ConsiderPassingBaseTypesAsParameters"), System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Design", "CA1006:DoNotNestGenericTypesInMemberSignatures")]
		public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expression)
		{
			if (expression == null)
				throw new System.ArgumentNullException("expression");

			var negated = Expression.Not(expression.Body);
			return Expression.Lambda<Func<T, bool>>(negated, expression.Parameters);
		}

		/// <summary>
		/// Combines the first expression with the second using the specified merge function.
		/// </summary>
		static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
		{
			// zip parameters (map from parameters of second to parameters of first)
			var map = first.Parameters
				.Select((f, i) => new { f, s = second.Parameters[i] })
				.ToDictionary(p => p.s, p => p.f);

			// replace parameters in the second lambda expression with the parameters in the first
			var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

			// create a merged lambda expression with parameters from the first expression
			return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
		}

		class ParameterRebinder : ExpressionVisitor
		{
			readonly Dictionary<ParameterExpression, ParameterExpression> map;

			ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
			{
				this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
			}

			public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
			{
				return new ParameterRebinder(map).Visit(exp);
			}

			protected override Expression VisitParameter(ParameterExpression p)
			{
				ParameterExpression replacement;

				if (map.TryGetValue(p, out replacement))
				{
					p = replacement;
				}

				return base.VisitParameter(p);
			}
		}
	}
}
